The present invention relates to an improved overlay or transfer type of negative working color proofing system which is developable with water alone. It has long been desired in the art to produce color proofing systems which are water developable. Water developability renders developer effluent more ecologically acceptable and provides a less hazardous working environment. Water developable color proofing systems are known in the art. These typically employ a combination of certain water soluble diazonium salts in conjunction with water soluble binder resins. While such materials are truly water developable, they are disadvantageous because fingerprint resistance is low. This invention improves upon such prior systems such as taught in U.S. Pat. No. 4,469,772 which specifies a combination of the methane sulfonate and mesitylene sulfonate salts and a water soluble resin binder. The present invention provides a combination of a water soluble diazonium salt and a water insoluble but water swellable resin binder.
In the field of reprographics, it is desirable to produce a color proof to assist a printer in correcting a set of photomasks which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image, and should neither gain nor lose color. Visual examination of a color proof should reveal any defects on the photomask: the best color rendition to be expected from press printing of the material; the correct gradation of all colors and whether grays are neutral; and the need, if any, for subduing any of the colors and/or giving directions for altering the film photomask before making the printing plates.
Color proofing sheets for multicolored printing have heretofore been made by using a printing press proof which requires taking all the steps necessary for actual multicolor printing. Such a conventional method of color proofing has been costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of pressproofs. There are three known types of photographic color proofing methods, namely, the surprint type, the overlay type and the transfer type.
In the overlay type of color proofing, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and placed on a white sheet to produce a color proof. Advantages are that it is quick and can serve as a progressive proof by combining any two or more colors in register.
In the surprint type of color proofing method, a color proofing sheet is prepared by successfully producing images of different colors from different color separation fills onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An example of this approach is described in U.S. Pat. No. 3,671,236. An advantage of the surprint type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles actual printing and eliminates the color distortion inherent in overlay systems.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are know as shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed to a photographic transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material U.S. Pat. No, 3,574,049 provides a transfer process for printing a design on a final support which comprises (a) printing a design onto a temporary support, (b) superimposing the temporary support and the final support, (c) applying heat and/or pressure to the superimposed structure formed in (b), and (d) separating the temporary support from the final support which retains the printed design. The affinity of the design for the temporary support is lower than its affinity for the final support.
In U.S. Pat. No. 3,721,557, a method of transferring colored images is claimed which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor member and, subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer.
This invention employs a water soluble diazonium salt which in the preferred embodiment is a polycondensation product of 3-methoxy-4-diazodiphenylamine sulfate and 4,4'-bis-methoxymethyl diphenylether precipitated as the chloride salt or the methane sulfonic acid salt, formulated with various water soluble and certain water insoluble but swellable resins to prepare a completely water developable overlay or, when overcoated with adhesive and transferred by lamination, a one piece proofing film. Additionally, since the preferred formulation utilizes water insoluble, water swellable resin binders, the films have the advantage of excellent fingerprint resistance.